Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, a radiation imaging system, and a storage medium.
Description of the Related Art
It is possible to acquire a radiation image of an object by irradiating the object with radiation and detecting the intensity distribution of the radiation transmitted through the object. The most common method as a method of acquiring a radiation image is to combine a so-called “fluorescent screen” (or “intensifying screen”), which emits fluorescence upon irradiation with radiation, with a silver halide film first and then irradiate them with radiation through an object. With this operation, the fluorescent screen converts the radiation into visible light to form a latent image of the object on the silver halide film. Thereafter, chemically treating the silver halide film on which the latent image of the object is formed can obtain a visible image of the object (a radiation image of the object) on the silver halide film. The radiation image obtained by such a radiation image acquisition method is an analog photograph, which is used for image diagnosis, examination, and the like.
On the other hand, it has begun to use a computed radiography apparatus (to be referred as a “CR apparatus” hereafter) using an imaging plate (to be referred to as an “IP” hereinafter) coated with a photostimulable phosphor as a phosphor. In this CR apparatus, secondarily exciting the IP primarily excited by irradiation with radiation by using visible light such as red laser light will generate light called photostimulable fluorescence. Detecting this fluorescence by a photosensor such as a photomultiplier can acquire image data (radiation image data). It is possible to output a visible image to a photosensitive material or CRT based on the image data. Although a CR apparatus is a digital imaging apparatus, since it requires an image formation process, that is, reading by secondary excitation, it can be called an indirect imaging apparatus. The reason why this apparatus is called an “indirect” apparatus is that it cannot display an acquired captured image (radiation image) in real time like a technique of acquiring the above radiation image as an analog photograph (to be referred to as an “analog photography technique” hereinafter).
There has recently been developed an apparatus which acquires a digital radiation image by using, as an image receiving unit, a photoelectric conversion unit (an imaging device such as a CCD) having pixels, each including a minute photoelectric conversion element and a switching element, arrayed in the form of a matrix. Such apparatuses are disclosed as radiation imaging apparatuses, each having a phosphor formed on a CCD or amorphous silicon two-dimensional imaging device, in U.S. Pat. Nos. 5,418,377, 5,396,072, 5,381,014, 5,132,539, and 4,810,881. This apparatus can display an acquired radiation image in real time and hence can be called a direct digital imaging apparatus.
In this case, an indirect or direct digital imaging apparatus is advantageous over an analog photography technique in that, for example, it can eliminate the need to use films, increase the amount of information acquired by image processing, and make a database. A direct digital imaging apparatus is advantageous over an indirect digital imaging apparatus in promptness or the like. The advantage of promptness, in particular, is that it is possible to, for example, instantly display a radiation image obtained by radiation imaging on the spot. That is, for example, this advantage is effective in an emergent medical situation.
In a direct digital imaging apparatus using an imaging device such as a CCD as an image receiving unit like that described above, the gain of each pixel of the imaging device is not constant. For this reason, in order to generate a uniform output image relative to an input image to the imaging device, it is necessary to perform gain correction for each pixel. Imaging for gain correction is called calibration, which is generally performed by the user at predetermined intervals. More specifically, first of all, the gain of each pixel of the imaging device changes with time due to the influences of usage environment conditions and the like. To acquire a high-quality output image, therefore, it is preferable to perform proper calibration for each startup of the imaging apparatus in accordance with the corresponding situation.
In calibration, first of all, the apparatus irradiates the entire effective imaging area of the imaging device with radiation upon removing any object (to be referred to as a “foreign substance” hereinafter) which makes it difficult to irradiate a radiation detector with a uniform dose of radiation. The apparatus stores the image obtained at this time (to be referred to as a “gain image” hereinafter). An object is then placed, and the apparatus performs actual radiation imaging (clinical imaging). The apparatus performs correction (to be referred to as “gain correction” hereinafter) of variation in the gain of each pixel of the image (clinical image) obtained by this operation by using the gain image stored in advance. Assume that when performing gain correction, the apparatus has captured a gain image with an improper dose of radiation or in the presence of a foreign substance. In this case, the apparatus may not properly correct the captured image by gain correction.
Japanese Patent Laid-Open Nos. 2001-351091 and 2010-12105 disclose a technique of determining whether a gain image is proper by comparing it with a gain image properly captured in the past.
If, however, a foreign substance which is mixed at the time of imaging operation is a low radiation attenuating substance, corresponding data is buried in noise according to conventional techniques. This makes it difficult to properly discriminate whether the imaging operation has been performed with the foreign substance being mixed.